I think you'll find superb pictures on A.net taken by either lens.
Personally, I went for the 100-400mm. This is a very sharp lens through the range. Don't know where you heard about blurring. Also, the IS is a big plus - it will let you shoot at much slower shutter speeds handheld which is great for poor light or panned shots. Realistically, the IS is worth 2 stops over a non IS lens.
50 - 500? well I've always felt that this equated to a very heavy "standard" lens
I would rather carry a 2nd, shorter lens in addition to the 100-400.
Ultimately, I think the sort of photography you do should be the deciding factor - is a 10x zoom that useful to you? How often will you use it at the shorter range? Is photography in poor light important to you?
Colin K. Work, Pixstel